Knowledge of reservoir behavior is important in planning and carrying out secondary and enhanced recovery processes in oil reservoirs. The use of chemical and radioactive tracers for reservoir evaluation has been thoroughly developed over the past few decades. In tracer studies, an identifiable tracer material is injected through one or more injection wells into the reservoir being studied. Water or other fluid is then injected to push the tracer to one or more recovery wells in the reservoir. The output of the recovery wells is monitored to determine tracer breakthrough and flow through the recovery well(s). Analysis of the breakthrough times and flows yields important information for use in carrying out the secondary or enhanced recovery process.
Most of the previously used tracers have been radioactive compounds. Output from recovery wells has been monitored for radioactivity to determine tracer breakthrough and flow.
In recent years, environmental and safety considerations have drastically impacted the use of radioactive tracers, and there has been a need for effective nonradioactive tracers for reservoir studies.
Nonradioactive tracers have been proposed, for example as described in U.S. Pat. No. 3,003,856 to Boyd, but have disadvantages in that the amount of tracer injected needs to be quite high in order to be detected during analysis of output from the recovery wells. Additionally, many chemical tracers are subject to reaction with materials in the reservoir, hindering their effectiveness.
There has been a need for chemical tracers that are safe, readily available, environmentally acceptable and detectible in very low amounts, so that small amounts can be injected and yet breakthrough and flow can be accurately measured. Such tracers are provided by the present invention.